Recently, as a highly efficient method for compressing and coding images, MPEG (Moving Picture Experts Group) system has been widely used and MPEGs 1, 2 and 4 have been internationally standardized. MPEG system removes redundancy in the spatial direction and the temporal direction from an image signal and compresses and codes the image signal. At this time, when the compression rate is small, the image quality of the decoded image signal hardly deteriorates. However, when the compression rate becomes higher, coding noises may be noticeable and the image quality may be deteriorated.
Typical examples of the coding noise in MPEG system include block noise. The block noise is lattice noise occurring when continuity of image signals between neighboring blocks is impaired when the image signals are compressed and coded in an 8×8 pixel block-shaped region and these compressed and coded image signals are decoded. Thus, since the block noise has regularity, it is perceived more easily as compared with random noise. Particularly, the block noise tends to be noticeable in a flat image region with a low spatial frequency and with little change in brightness. The block noise is known to be recognized clearly in a region which is relatively flat and includes gradation, for example, an image of sky, human skin, or the like.
As a block noise reduction method, Japanese Patent Unexamined Publication No. H10-191335 proposes a method of firstly detecting a boundary between blocks and then performing smoothing processing such as low-pass filter processing to image data in the boundary between blocks.
However, in the above-mentioned block noise reduction method, it is difficult to reliably distinguish the boundary between blocks from a mere contour portion of image. When smoothing processing is performed to a counter potion that is mistaken as the boundary between blocks, the resolution is deteriorated so as to cause the counter portion of the image to appear blurred.